The Factor IX activator (Factor XIa) has been isolated from bovine serum, and has been shown to be an enzyme possessing esterase activity and subject to inhibition by DFP and PMSF. It catalyzes the conversion of Factor IX to Factor IXa in the presence of calcium and other divalent cations. The goals of this project are the following. First, to purify the Factor IX activator (Factor XIa) to homogeneity from bovine serum; significant progress has been made toward this goal, and it should be possible to achieve in the first year or two of this project. Second, to determine the amino acid sequence around the active site serine of the Factor IX activator (Factor XIa), to demonstrate that it is similar to but not identical with other proteases of the trypsin-chymotrypsin-thrombin family. Third, to demonstrate by primary structure analysis the changes in the Factor IX molecule which are brought about by the activation of Factor IX to Factor IXa under the influence of the Factor IX activator and divalent cations. And fourth, to perform experiments designed to delineate more precisely the origins of the molecule frequently designated Factor XIa, which catalyzes the conversion of Factor IX to Factor IXa. These latter experiments will be designed to demonstrate or exclude the role of cells in the evolution of the Factor IX activator during clotting; to demonstrate or exclude the role of Factor XI in the evolution of the Factor IX activator; and to demonstrate directly that the Factor IX activator is or is not derived directly from purified, unactivated bovine Factor XI purified from plasma. If cells should be shown to participate in the formation of Factor IX activator, and if the cells involved should turn out to be platelets, a connection would thus be established between platelet aggregation and fibrin formation in the evolution of the thrombus.